It is known to provide containers, especially bottles, made of a thermoplastic such as polyethylene terephthalate PET, polyethylene naphthalate PEN, or another thermoplastic, with an internal partition. Such a partition may be complete, i.e. it may extend over the entire height of the container, in order to divide the internal volume of the container into two chambers containing two respective liquids which must not be mixed together before being poured. As shown in FIG. 1 of the appended drawing, the partition 1 may also be partial, i.e. it may extend only over part of the length of the body 2 of the container 3, in order to retain (bracing) two opposed portions of the body of the container, for example in order to maintain two gripping recesses 4 hollowed out in the body of a container filled with a pressurized liquid, which recesses, even if they were not reinforced, would risk being deformed due to the effect of the pressure of the liquid.
It is known to manufacture such containers from preforms which themselves are provided with a totally or partially longitudinally extended internal partition depending on the type of final container to be obtained. Such a preform 5 with a partially longitudinally extended internal partition 6, intended for the manufacture of the container in FIG. 1, is illustrated in FIGS. 2 and 3 of the appended drawing, respectively as a side view in FIG. 2 and, in the case of FIG. 3, in a cross-sectional top view on the line II--II of FIG. 2. It has a general structure similar to that of a partitionless preform with a neck 7 and a thick-walled body 8 which are conventionally shaped. The internal partition 6 has a thickness appreciably smaller than that of the thick wall of the body.
When this known preform, after it has been heated, is subjected to a stretching or stretch-blowing process in order to obtain a container, the internal partition stretches, as does the wall of the body: the longitudinal stretch ratio of the partition is identical to that undergone by the body (for example, in order to be specific, typically 2.22 to 3.5 for the PET grades currently used on an industrial scale). On the other hand, when the body of the container to be obtained has to have a localized transverse necking--for example distance between the bottom of the gripping recesses 4 of the container of FIG. 1--, the transverse stretch ratio of the partition is locally very much less than the transverse (or perimetric) stretch ratio of the body of the container: in order to be specific, the transverse or perimetric stretch ratio of the body is generally at least about 4, while the transverse stretch ratio undergone by the partition remains here less than 4.
As a result, there is a disadvantageous difference between the properties of the material of the partition and of the material of the body of the container: since its material has not undergone sufficient transverse elongation, in order to orient the molecular chains in a complementary fashion the partition is subject to deformation when the container is subsequently exposed to severe conditions of use and the partition is no longer capable of effectively ensuring its bracing function.